The present invention is particularly useful for transmission and reception of wireless cellular communications. The invention is suited for use in common frequency bands, such as 800-960 MHz or 1700-2200 MHz. While most common base station antennas cover wide sectors around the base station, the intention of the present antenna is to cover very narrow sectors with dual polarization and pencil beam. Although the antenna is particularly useful in cellular infrastructure, it can also be used in other types of radio communication links and at other frequencies, providing very high gain and dual polarization.
Base stations used in cellular and other wireless communication systems, especially those supporting mobile units, as well as the mobile units themselves, suffer from the well-known problem of multi-path fading. One means to overcome this problem is the use of receive and transmit diversity, which together are also known as diversity reception. In diversity reception, two uncorrelated fading path signals propagate between the signal source and the receiving party. With two uncorrelated signals, each going through a different fading mechanism at any time, there is a good chance that at least one path is received strong enough for data subtraction at any time. One of several known diversity techniques is polarization diversity, where two orthogonally polarized elements provide uncorrelated propagation paths, both in receive and in transmit modes. The antenna of the present invention relates to mutually orthogonal, linearly polarized elements, which can be set to either vertical/horizontal (or 0°/90°) or +45°/−45° relative to the Earth's horizon. Such an antenna is known as cross-polarized or dual-polarized.
The radiating element of a parabolic reflector dish antenna can be constructed of slant +45° and −45° oriented dipoles. Such an arrangement of a pair of crossed dipoles whose mechanical centers are co-located and their linear polarization axes are at 45° with respect to the vertical axes of the antenna, is well known in the art. A dipole radiator located at the focal point of a parabolic reflector dish does not provide the optimal feed element for such an arrangement due to different radiation patterns for E and H planes. An improved dipole radiation scheme is provided by mounting the feeding half-wavelength cross dipole at the mouth of a shallow cylindrical metal cavity. U.S. Pat. No. 4,109,254 and U.S. Pat. No. 4,005,433 disclose the use of crossed dipoles located at the mouth of a circular cavity and feeding a parabolic reflector with coaxial feeds coming through the dipole base where the balanced-to unbalanced transformers (BALUNs) are located. With this arrangement, one or more annular chokes may be provided around the cavity to further shape-feed radiation pattern and reduce the side lobes and back lobe of the composite radiator.
In contrast to the mechanically machined dipole elements and BALUN used in these previous techniques, the present invention discloses a unique structure of lower cost printed circuit board (PCB) technology to implement the crossed-dipole feed elements of a dish reflector antenna.
A printed cross-dipole radiator is described in Japanese patent application JP 2001/168637, which shows a miniaturized cross dipole using printed circuit board. (PCB) technology. However, neither this nor any other solutions known to the inventors disclose a true crossover feeding line arrangement of orthogonal radiating element boards that are DC-short-circuited to ground, thereby providing reliable lightning protection. Nor do these prior art solutions provide perpendicular PCB dipoles mounted within a metal cap-shaped cavity, feeding a parabolic dish reflector.
The use of a parabolic reflector dish is not common in the cellular communication industry due to size and general appearance of such antennas. The large size is a consequence of the physical requirement that the diameter of the dish be at least four times the maximum wavelength in use. With a maximum cellular band wavelength of 37 cm, the minimum dish diameter would be 1.4 meters or in practice 2 meters. The visual impact of cellular base station towers on communities and individuals has become a major concern. It is thus a vital necessity to reduce the size or (if physically impossible) the visual impact of the base station towers and antennas on their surroundings.
The common means for reducing the visual impact, as well as the wind load and weight, of a parabolic dish reflector is to use a metal grid, such that the large dish will appear to be substantially transparent. U.S. Pat. No. 5,421,376 and U.S. Pat. No. 5,456,759 disclose a collapsible parabolic dish made from rigid ribs and metalized mesh fabric. These prior art patents use very fine cross woven metalized mesh which might be light but certainly not transparent. By contrast, the present invention discloses a parabolic reflector which is field assembled of four identical quadrants while each is made of rigid ribs and relatively very large spacing cross woven metal grid which are larger than the useful wavelength over 10 (λ/10). This quality is enabled since all metal wires of the reflector of this invention are parallel to the electric field lines radiated towards the reflector.
U.S. Pat. No. 4,893,132 describes a parabolic dish antenna which is assembled out of four quadrants. The present invention presents a parabolic reflector assembled of four quadrants as well but as opposed to previous art these quadrants are made of cross woven metal mesh where all wires in any of the quadrants are parallel to the wires in all other three quadrants thus making all quadrants identical and simpler to manufacture.
Parabolic dish reflector antennas used for cellular communications are vertically linearly polarized with the reflector being made of parallel, spaced metal rods, or fins, spaced apart a distance that is less than the wavelength divided by 10 (λ/10). U.S. Pat. No. 5,191,350 discloses a single vertical polarization antenna using a parabolic reflector having very large openings. The reflector presented in that patent is made out of parallel metal rods so as to support a single polarization only and can be assembled of two identical sections. By contrast, in the present invention the parabolic dish reflector is made of a cross-woven metal grid with large openings, enabling dual cross polarization radiation.
It is an intention of this invention to provide a parabolic dish reflector antenna that is dually polarized for diversity reception purposes while causing minimal visual disturbance for use with cellular base stations and repeaters. The parabolic reflector dish is built from four identical quadrants that are made from cross-woven metal wire with large openings and that can be assembled in the field to compose a full reflector wherein all grid wires are parallel to the cross-polarized electrical fields.
It is further the intention of this invention to provide a cross-dipole feed for illuminating the parabolic reflector dish antenna, and which is implemented by printed circuit board technology (PCB), enabling lower cost assemblies.
The present invention also discloses the application of a parabolic dish reflector antenna as a high gain dual polarization antenna in the cellular infrastructure. The practical requirements of base station and repeater antennas, known to those familiar with the cellular infrastructure industry, prevent the use of higher gain dual polarization dish antennas due to large size, heavy weight, high wind load stress on the tower or pole, and visual stress on nearby communities served by the cellular network.
The antenna structure disclosed by this invention makes such high gain dually polarized antennas applicable for cellular infrastructure. Other features and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description constructed in accordance with the accompanying drawings wherein:
It is an object of the present invention to provide an antenna for cellular base stations that supports dual polarization signaling for signal combining and polarization diversity.
It is a further object of the present invention to provide an antenna that is capable of very high gain with narrow beam width on both azimuth and elevation with very low side lobes.
It is a further object of the present invention to provide a dish reflector antenna that has very low visual impact on the environment and that has reduced wind load due to its mesh structure.
It is a further object of the invention to provide an antenna that can be field assembled to minimize transportation expenses.
It is a further object of the present invention to provide orthogonally-arranged printed dipole structures including crossover feeding lines and BALUNs, collocated and having the same phase-center within a circular cavity.
It is a further object of the present invention to provide a dielectric stud rigidly supporting printed circuit board dipoles location at the center of a conductive circular cavity.
It is a further object of the present invention to provide an inherent DC grounding arrangement for the radiating elements, enabling lightning-induced currents to be shunted to ground.
These and other objectives of the invention are provided by an improved antenna system for cellular base transmission stations.